Light emitting diodes (LEDs) are generally more energy efficient, more reliable and have longer lifetimes than other types of lighting. One performance measure of an LED is its photometric efficiency, e.g. the conversion of input energy into visible light. Photometric efficiency is inversely proportional to the junction temperature of an LED. Junction temperature also affects the operational lifetime of LEDs. Accordingly, keeping the LED junction temperature cool is an important consideration in the design of LED devices.
Traditionally, heat dissipation of LEDs was provided by the lead wires of the LED itself. However, this technique is inefficient and limits the efficiency of LED devices. Another method for controlling LED junction temperature uses a heat sink slug to draw heat away from the LED. An example of such an apparatus is described in U.S. Pat. No. 6,274,924 to Carey et al., issued Aug. 14, 2001. An LED die is attached to the heat sink slug using a thermally conductive material or submount. The heat sink slug is inserted into an insert-molded leadframe. The heat sink slug may include a reflector cup. Bond wires extend from the LED to metal leads on the leadframe. The metal leads are electrically and thermally isolated from the slug. An optical lens may be used to focus the light emitted from the LED. This apparatus is useful for dissipating heat from the LED, however it requires that the heat be dissipated to air. This problem becomes exacerbated with high wattage LEDs and multiple LED devices where heat generation is greater. A solution to the external heat dissipation is not provided by the apparatus of Carey et al.
Control and focus of the light emitted from an LED is typically provided using a collimator such as those described in U.S. Pat. No. 6,547,423 to Marshall et al., issued Apr. 15, 2003. A collimator uses a lens and refractive walls to focus the light emitted from an LED. An LED and collimator combination yields a high level of efficiency in terms of control of emitted light or luminous flux.
The aiming of individual light sources so that the object or area of interest is properly lit is an important consideration. A known method of aiming individual light sources is an arrangement commonly referred to as a gimble ring. Gimble rings are known in the art and are commonly used in track lighting. Gimble rings work well with incandescent lights and other light sources that do not depend on a thermal circuit at the back of the lighting assembly. However, gimble rings are not suitable for light sources that require a thermal circuit at the back because the ring arrangement lacks the required surface area. Further, gimble ring-type arrangements are not appropriate for use in small spaces, for example, where clearance around the light source is limited or where several light sources are to be used close together.
Thus, it would be desirable to have a lighting assembly for an LED that provides adequate heat dissipation for single LED applications, high wattage LEDs and multiple LED devices. Also desirable is a lighting assembly for LEDs and other light sources requiring a thermal circuit at the rear which provides for the aiming of individual light sources.